A plasma etching apparatus is used in, e.g., a manufacturing process of a semiconductor device such as IC (integrated circuit), LSI (large-scale integration) and the like. In such a plasma apparatus, there is employed a power supply device for plasma generation that is a high frequency power supply device used for generating plasma. A conventional high frequency power supply device will be described with reference to FIGS. 1 and 5. FIG. 1 is a block diagram showing a functional configuration of a high frequency power supply device in accordance with an embodiment of the present invention, but it is identical to the conventional high frequency power supply device except for a function of a control unit. FIG. 5 is a schematic view showing signal waveforms of the conventional high frequency power supply device.
As shown in FIG. 1, the conventional high frequency power supply device includes an oscillation unit 11, a modulation unit 12, a level adjustment unit 13, a power amplifier 14, an output power detection unit 15 and a control unit 16. The level adjustment unit 13 includes a level adjustment circuit 13a and a D/A (digital-to-analog) converter 13b. The output power detection unit 15 includes a directional coupler 15a, a detector 15b and an A/D (analog-to-digital) converter 15c. 
As shown in FIG. 1, a RF (radio frequency) signal 11s that is a high frequency signal sent from the oscillation unit 11 is pulse-modulated by the modulation unit 12. A power level of the pulse-modulated signal is adjusted by the level adjustment unit 13 and the level-adjusted signal is inputted to the power amplifier 14. An output of the power amplifier 14 is outputted to a plasma load 20 through the output power detection unit 15.
In the output power detection unit 15, the detector 15b detects an output power Pf of the power amplifier 14 extracted by the directional coupler 15a, and the A/D converter 15c converts the detected power into a digital signal and outputs the digital signal to the control unit 16.
The control unit 16 obtains a difference between the output power detected by the output power detection unit 15 (i.e., the digital signal from the A/D converter 15c) and a set power that is previously set, and controls a level adjustment value to be outputted to the level adjustment unit 13 such that the difference becomes zero. The control unit 16 outputs a level control signal 16s2 to the level adjustment unit 13. The D/A converter 13b converts the level control signal 16s2 into an analog signal and outputs the analog signal as the level adjustment signal 13bs to the level adjustment circuit 13a. 
As such, the control unit 16 controls the output power of the high frequency power supply device to become a constant value by controlling the level adjustment circuit 13a. The level adjustment circuit 13a adjusts the output power by using a circuit of a variable attenuator or the like.
FIG. 5 shows time waveforms of the respective units. In FIG. 5, (a) depicts a waveform of the output power Pf, (b) depicts a waveform of the modulation signal 16s1, and (c) depicts a waveform of the level adjustment signal 13bs. The output power Pf is a high frequency signal and an envelope curve of the high frequency signal is shown in FIG. 5. As such, the waveform of the output power Pf is formed by pulse-modulating the RF signal 11s from the oscillation unit 11 by using the modulation signal 16s1.
A method of adjusting a level of the output power in the conventional high frequency power supply device will be described. First, a conventional output power level adjusting method will be briefly described. The control unit 16 detects the high frequency output power Pf through the output power detection unit 15 from a time point at which the modulation signal 16s1 is turned on. Marks ∘ shown in FIG. 5 are detection points of the output power Pf. Next, the control unit 16 compares an average value of the detected output power Pf with a set power, and calculates a level adjustment value such that the difference between the average value and the set power falls within a predetermined range. A value of the level adjustment signal 13bs is changed and outputted at a timing at which the modulation signal 16s1 is turned off. The changed value is reflected in a subsequent pulse output. In such a high frequency power supply device that outputs a pulse-modulated high frequency power, it is general to perform, between pulses, a control of an output power level because there may be a case where a pulse width is as short as several μs.
The conventional output power level adjusting method will be described in detail with reference to a flow chart of FIG. 6. FIG. 6 is a flow chart showing the conventional output power level adjusting method. The adjustment of an output power level is controlled by the control unit 16. First, a set power and an allowable power range are set in an initial setting in step S101. The allowable power range is an allowable difference value between the output power Pf and the set power.
Next, the high frequency power supply device is operated and it is examined whether or not the modulation signal 16s1 has been turned on, i.e., whether or not the output power Pf has been outputted in step S102. If the modulation signal 16s1 is not in an on state (NO in step S102), the device waits until the modulation signal 16s1 is turned on. If the modulation signal 16s1 is turned on (YES in step S102), a value of the output power Pf (e.g., Pf1) at that moment is obtained in step S103. Thereafter, it is examined whether or not the modulation signal 16s1 has been turned off, i.e., whether or not the output power Pf has been turned off in step S104. If the modulation signal 16s1 is not in an off state (NO in step S104), the flow goes to step S103, and a value of the output power Pf (e.g., Pf2) at that moment is obtained.
If the modulation signal 16s1 is turned off (YES in step S104), an average value of the obtained output power Pf (Pf1, Pf2, . . . ) is calculated in step S105, and the average value of the output power Pf and the set power are compared to each other in step S106.
If a difference between the average value of the output power Pf and the set power is within the allowable power range (YES in step S107), the flow returns to step S102. If the difference between the average value of the output power Pf and the set power is not within the allowable power range (NO in step S107), a level adjustment value N is calculated based on the difference between the average value of the output power Pf and the set power in step S108. For example, if the average value of the output power Pf is larger than the set power while exceeding the allowable power range, the level adjustment value N is calculated to decrease, and if the average value of the output power Pf is smaller than the set power while exceeding the allowable power range, the level adjustment value N is calculated to increase.
Next, the level adjustment value N is updated in step S109, and the flow returns to step S102. By updating the level adjustment value N, a magnitude of the level adjustment signal 13bs outputted to the level adjustment circuit 13a is updated.
In Japanese Patent Application Publication No. 2002-270574, there is disclosed a plasma etching apparatus that applies a pulsed high frequency power to a vacuum chamber in which a plasma etching is performed on a wafer.
As described above, in the conventional output power level adjusting method, a level adjustment value is set such that a difference between an average value of the detected output power Pf and the set power falls within a predetermined range in an off state between modulation pulses. By doing so, an average output power of a subsequent modulation pulse is controlled. However, impedance of a plasma load is not always constant and changed depending on an operation state of the plasma load even during an on-state of the modulation pulse. If the impedance of the plasma load is changed, the characteristic of the power amplifier 14 is changed and a value of the output power Pf is separated away from a value of the set power.
That is, as shown in FIG. 5, when the modulation signal 16s1 is turned on and the power Pf is supplied to the plasma load, the plasma load starts to operate. However, after the operation of the plasma load, the state of the plasma load is not constant and impedance of the plasma load is changed. For this reason, when controlling only an average output power as in the conventional way, the output power Pf is changed in an on-state of the modulation pulse, so that a value of the output power Pf is separated away from the set power value. The object of the present invention is to provide a power supply device for plasma generation which can prevent a value of the output power from being separated away from the set power value by suppressing fluctuations of the output power in an on-state of the modulation pulse.